1. Field of the Invention
The present invention relates to a light sensing apparatus and the method for luminaire calibration; more particularly, the light intensity for every LED can be measured in sequence by a frequency modulation scheme for further calibration.
2. Description of Related Art
With the usage of light emitting diodes (LED) in common luminaires, the illuminating quality of a plurality of LEDs is getting much attention. In the prior arts, some of them utilize a detection mechanism to modulate the chromatic characteristics of the luminaire automatically or manually.
Since the illuminating efficiency of the LED will be influenced often by the ambient circuit, driving current, the difference between the chips, or any change of the environmental temperature, a uniform illumination could not be obtained using LEDs as a light source. A detection scheme was provided in the prior art, by means of a modulation based on feedback information from the light emitted by the LEDs, for overcoming the drawback of the conventional LED luminaire.
The background art, such as the U.S. Pat. No. 6,127,783 (issued on Oct. 3, 2000), had provided a method for electronically adjusting the color balance of LED luminaires. As usual, the disclosed white light emitting luminaire includes a plurality of LEDs mixed with red, green and blue colors, and each of the colors uses separate power supply. Thus, a calibration mechanism is needed to adjust the output of the LED. For example, the output of each color is adjusted in response to the comparison between the measurement of the output and a default value, and it's to determine the power supplying the LED chip thereby.
Reference is made to FIG. 1, where an LED luminaire is shown to have a controller 17 used to adjust the current and lighting mode of the LEDs 10 by the regulators 11. A power converter 18 is used to convert the power to supply the current for the LEDs 10. The LEDs 10 can be the individual red, green and blue color, and then the colors are mixed as a white light through a diffuser 12.
In order to achieve the adjustment, a light sensor 14 shown in FIG. 1 is utilized. The light sensor 14 firstly measures the light emitted by the LEDs 10 through the diffuser 12. The sensed signals will be fed back to the controller 17, and the controller 17 performs a comparison between the feedback signals and a default value configured by an external control 16. Accordingly, the regulators 11 are used to adjust the current for each LED 10.
In another aspect, U.S. Pat. No. 6,495,964 issued on Dec. 17, 2002 further disclosed a scheme that instantly increases the drive current for the LED to compensate for the unstable state occurring after switching on or switching off the LED. A sensor made by photodiode is utilized to periodically measure the intensity of each LED in sequence. Since it is sufficient to compare and adjust the current for LED in a very short time, the LED-to-be-measured will be turned on in an instant period and the others are simultaneously turned off.
However, the luminosity of the LED will be insufficient or unstable in the very short time as the LED is turned on or turned off. In order to compensate for the luminosity, some conventional technologies used an extra circuit to increase the current instantly around the time to turn on or turn off the LED. Referring to FIG. 2, wherein the horizontal axis represents time and the vertical axis means current. The current shown as the numeral 20 in the diagram has a sudden rise around the time as the LED is turned off, namely the compensation pulses occurred before turning off the LED and after turning on the LED. That is to function as compensation by suddenly increasing the current.